The present invention relates to a device for machining a large workpiece and, more particularly, to a device for machining the rotatable circle that supports the blade of an earth grader.
Earth graders typically support a blade that may be raised and lowered to adjust the depth of cut of the grading edge, and may be rotated about a vertical axis to adjust the angle of the grader edge with respect to the direction of travel of the grader, thus adjusting the width of the cut. The blade is mounted on a removeable, internally-toothed "circle", the teeth on the circle meshing with teeth on a rotatable turntable carried by the grader. The circle is typically made of cast metal and is on the order of 40 inches in diameter.
After a period of use, the teeth on the circle become worn and the circle, itself, worn outof round, permitting play of the circle with respect to the grader turntable, which greatly affects the accuracy with which the grader blade may be adjusted. Consequently, the teeth of the circle are periodically reconditioned to ensure a minimum of play and the greatest amount of grading accuracy.
To recondition the grader circle, the worn tooth segments are removed and new teeth are welded to the interior of the circle. After such welding, the upper and lower surfaces of the new teeth must be ground parallel to one another, and the interior surface of the circle machined to be concentric with the crest of the teeth and perpendicular to the upper and lower surfaces thereof. The accuracy of machining the reconditioned circle is critical to insure that the circle will rotate freely, with minimal play, when it is replaced on the grader and to permit precise adjustment of the grader blade.
While accurately machining large workpieces is typically a difficult procedure, the machining of the wear surfacs of the grader circle is further complicated because the welding of the teeth to the circle serves to further distort the geometry of the circle. Previously, grinding of the three wear surfaces has been a time consuming and difficult process because of both (1) the accuracy required, particularly to machine the parallel surfaces of the reconditioned teeth, and (2) the large size of the workpiece to be machined.
Accordingly, it is the principal object of the present invention to provide an apparatus that accurately machines the reconditioned circle of an earth grader.
A further object is to provide an apparatus that simultaneously machines the parallel surfaces of a circular workpiece.
More particularly, it is an object to provide an apparatus that quickly and accurately machines each of the three wear surfaces of a reconditioned grader circle.
It is an additional object to provide such a device that is simple to set up and operate.
These objects, as well as others that will become apparent upon reference to the accompanying drawings and following detailed description, are provided by an apparatus for machining a circular workpiece including a frame having support means for securing the workpiece to the frame. A center tube is rotatably supported by the frame so as to extend into the interior of the circular workpiece at substantially the center thereof so that a central axis defined by the tube is substantially perpendicular to a horizontal plane defined by the workpiece. The center tube supports diametrically opposed spindle arms, one spindle arm supporting a pair of rotatable grinding wheels at its outer end so that the grinding surfaces of such grinding wheels define planes substantially parallel to the plane defined by the workpiece. The other spindle arm supports a rotatable grinding wheel at its outer end that has its grinding surface defining a plane substantially perpendicular to the plane defined by the workpiece. Adjustment means is provided for moving the parallel grinding wheels in unison in a direction perpendicular to the plane defined by the workpiece, with a second adjustment means provided for adjusting the position of one of the parallel grinding wheels in such a direction. Adjustment means is also associated with the third grinding wheel for controlling the radial position thereof. The three grinding wheels are rotated with respect to their spindle arms and the center tube is rotated so as to move the grinding wheels about the circumference of the workpiece to simultaneously machine each of the three wear surfaces thereof. In a preferred embodiment, the grinding wheels are rotated by a series of rotatable shafts connected by drive belts, certain of the shafts being mounted eccentrically within rotatable bearing housings so that the drive belts may be adjustably tensioned by rotating the bearing housings.